ARM: remove unused arch_decomp_wdog()

With ARCH_HAS_DECOMP_WDOG removed from arch/arm/boot/compressed/decompress.c,
all the arch_decomp_wdog() definition at platform level is unneeded.
Remmove it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>